RFM2Pi Gateway: pass ServerDataBuffer a logger name

This fix address a design issue in the python script I realized thanks to this forum post:
http://openenergymonitor.org/emon/node/2246

I'm using classes for more re-usability, but class ServerDataBuffer relied on a RFM2PiGateway instance to work.

With this fix, it can now be called by itself.

If no logger name specified, it uses root logger (which AFAIU means it logs nothing unless explicitly specified). 

For the record, here's a hint about how to configure the logging for a library:
http://docs.python.org/2/howto/logging.html#library-config

I didn't do it this way, but if a shortcoming is identified in the future, this may be an alternative.